1. Field of the Invention
The present invention relates to a power transmission apparatus for vehicles which transmits a driving force from one transmission shaft to the other transmission shaft by hydraulic pressure generated in a hydraulic pump, more particularly, it relates to a power transmission apparatus for vehicles which, in a four-wheel drive vehicle, transmits the driving force from the transmission shaft on the front wheel side to that on the rear wheel side or vice versa to realize a four-wheel driving state.
2. Description of the Related Art
In recent years, a four-wheel drive vehicle is particularly highlighted as the vehicle which can realize comfortable driving regardless of the road conditions, natural conditions such as weather and driving conditions, because of its high running stability in specific surface conditions such as the snow and ravel roads and its superb driving stability during the high speed driving and at acceleration and deceleration also on ordinary roads.
Furthermore, recently, a so-called full-time four-wheel drive vehicle constructed always to obtain substantially the four-wheel driving state by having a power transmission apparatus, which changes power distributions to the front and rear wheels responsive to the rotational speed difference generated therebetween when it took place, is a preferred product.
As such a four-wheel drive vehicle, it is the general trend to provide the power transmission apparatus which distributes the driving force responsive to the rotational speed difference between the front and rear wheels in the mid-part of a transmission shaft between the front and rear wheels. As such a power transmission apparatus, there is one which utilizes hydraulic pressure generated in a hydraulic pump particularly a vane pump which can be easily made to be small in size, light in weight and having good durability.
The vane pump, as already, known is provided with a casing with an annular cam ring having deviation in thickness and side plates mounted on its sides and a short cylindrical rotor onto which a plurality of plate vanes are disposed substantially in equal intervals in a circumferential direction mounted movable in a radial direction. This vane pump is constructed to form a pump chamber surrounded by the outer surface of the rotor, the inner surface of the cam ring, and further, by these and side walls of the side plates, by containing the rotor rotatably coaxially in an inner space of the casing. By sealing working oil introduced into the pump chamber between the vanes adjoining each other, pressure is raised responsive to the rotation of the rotor.
The power transmission apparatus is so designed that, by fixing the rotor coaxially to the driving shaft connected to either of the front and rear wheels, and fixing the casing coaxially to the driving shaft connected to the other wheels, relative rotation corresponding to the rotational speed difference between the front and rear wheels is generated between the rotor and the casing. At this time, since the hydraulic pressure generated iii the vane pump chamber corresponding to the relative rotation or the rotational speed difference between the front and rear wheels acts to restrain the relative rotation between the rotor and the casing. The driving force responsive to the rotational speed difference between the front and rear wheels is transmitted from one of the front and rear wheels to the other via the hydraulic pressure, thereby producing a desired four-wheel driving state.
In a four-wheel drive vehicle, for example, in case of driving on bad roads such as the snow and gravel roads, it is desirable to obtain a rigid coupling state as much as possible to achieve stable running. Meanwhile, in case of driving on the paved road, it is desirable to obtain a relatively loose coupling state to change transfer characteristics of the driving force between the front and rear wheels so as to reliably prevent a tight corner braking phenomenon. Besides, it is also desirable to change the transfer characteristics not only responsive to the road conditions aforementioned, but also to various running states such as the vehicle speed, steering angle and braking.
In the aforesaid power transmission apparatus which makes hydraulic pressure of a hydraulic pump as a transmission medium of driving force, it is possible to change a transmission characteristic according to changing a pressure characteristic which is dependent upon the magnitude of oil-through resistance an the discharge side of the hydraulic pump. To accomplish this, a power transmission apparatus has been proposed, which is so configurated as to satisfy the demand for changing a transmission characteristic into one responsive to a driving state of a vehicle by arranging a variable throttle member driven by a solenoid at a mid portion of the discharge side oil passage and controlling current flowing to the driving coil of a solenoid on the basis of the detected results of a various kinds of state quantities related to the driving state of a vehicle, such as steering angle, vehicle speed and number of rotations of an engine and to successively adjust a throttle opening of the amiable throttle member.
But since such a configuration demands an appropriate throttle opening corresponding to the respective detected results of a plurality of state quantities and a control unit of a complicated configuration which successively controls current flowing to the driving coil on the basis of the results, and results in bringing about a high product cost. Also, at the same time, a possibility exists that an unexpected accident originated in malfunction may happen due to a high possibility of defective operation because of electrical disturbances.
The applicants of the present invention have proposed a power transmission apparatus for vehicles in Japanese Patent application No. 2149650 (1990) in order to solve such difficulties. In the following, an explanation will be given on the power transmission apparatus disclosed iii this Japanese Patent Application No. 2149650 (1990) as a conventional example.
FIG. 1 is a longitudinal sectional view showing essential parts of the conventional power transmission apparatus for vehicle.
As shown in FIG. 1, in this conventional power transmission apparatus, between a vane pump 3 generating hydraulic pressure responsive to a rotational speed difference between front and rear wheels, and an output shaft 2 coupling with one of front and rear wheels a holding cylinder 50 is interposed coaxially and coupled thereto through respective coupling flanges 51, 52. And at the inside of' a spool chamber 60 formed at a shaft center portion of the holding cylinder 50, a spool 6 is inserted movably along the shaft. The spool 6 is energized by two coil springs 60a, 60b of different length in the same direction and the spool 6 moves responsive to a driving coil 7 being circumferentially arranged at the outside of the holding cylinder 50, to configure a variable throttle member, opening/closing an opening end of a communicating hole 56 communicating with the discharge side of the vane pump 3 through an oil guiding hole 42 and an annular groove 37.
Current flowing to the driving coil 7 is changed in two steps, to produce two kinds of magnetic fields of different strength at the inside of the spool chamber. At the cutoff time of current, the spool 6 is at the position shown in the figure (opening position) pressed against a snap ring 60c attached to the spool chamber 60 by energizing force of a long coil spring 60a. And at a time of forming a weak magnetic field, the spool 6 moves by a distance S shown in FIG. 1 against the spring force of the coil spring 60a and stops at a position (middle position) where it contacts with another coil spring 60b. At a time of a more intense magnetic field the spool 6 moves against the spring force of both of the coil springs 60a, 60b and is restricted at a position (closing position) where it is pressed in the inside end of the spool chamber 60.
That is, according to aforementioned operation, the position of the spool 6 inside of the spool chamber 60 is variable in three ways. Since the opening area of the communicating hole 56, that is, oil through resistance at the discharge side of the vane pump 3, varies in three ways according to the movement of this spool 6, three kinds of transmission characteristics satisfactorily enough for practical use are realized. On the other hand, only two kinds of controlling is needed for adjusting the quantity of current flowing to the driving coil 7 in order to move the spool 6. Accordingly, configuration of the control unit for controlling the current control is simple, and one of the aforesaid difficulties is solved.
And, in the aforesaid Japanese Patent Application No. 2149650 (1990) a power transmission apparatus has been proposed, in which three kinds of transmission characteristics, by replacing the energizing of the spool 6 by two coil springs 60a, 60b, by the fact that the closing position, middle position and opening position are obtained respectively according to current flowing to both the coil springs 60a, 60b, current flowing only one coil spring 60a or 60b, and current cutoff to both of the coil springs 60a, 60b, thereby the aforesaid difficulties being solved by the proposal of this power transmission apparatus for vehicle.
In the conventional power transmission apparatus the spool 6 is positively restricted at the opening position where the spool 6 contacts with the inside end of the spool hole 60. However, at the middle position, the spool 6 is in a floating state supported by a balanced force between the spring force of the coil spring 60a, 60b and magnetic force generated by the driving coil 6. Accordingly, the position of the spool 6 tends to vary and there is a difficulty, that a transmission characteristic determined by the position of the spool 6.
Such a difficulty is more remarkable in the conventional configuration which adjusts the throttle opening of the variable throttle member by successively controlling current flowing to the solenoid. This produces a stabilized corresponding relation between the quantity of current flowing to the solenoid and the throttle opening is not obtained and transmission characteristics vary respectively at the respective throttle openings. As a result, there is the desirability of realizing a desired transmission characteristic corresponding to driving condition of a vehicle with high reliability.